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Author:
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Montero Alejo, A. L.; Fuentes, M. E.; Menéndez Proupin, Eduardo; Orellana, W.; Bunge, C. F.; Montero, L. A.; García de la Vega, J. M.
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Abstract:
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Optical properties of three kinds of zigzag 5,0 , 13,0 , and 9,0 single-walled carbon nanotubes
SWCNTs are studied using an approximate quantum mechanical method named complete neglect of differential
overlap, which distinguishes basis atomic orbitals with different azimuthal l quantum numbers
CNDOL . This method models the electron energy transitions and excited state charge distributions through a
configuration interaction of singly CIS excited determinants allowing the direct understanding of properties
related with the total electronic wave function of nanoscopic systems, projecting a reliable quantum mechanical
understanding to real life objects. The finite SWCNT’s structures were obtained by replicating the unit cells of
periodic SWCNTs and saturating the edge dangling bonds with hydrogens. The unit cell was previously relaxed
using standard density functional theory methods. The behavior of these SWCNTs were interpreted in the
framework of the CNDOL scheme by increasing the lengths of the tubes above 3 nm. As the nanotubes grow
in length, the position of excited states for each SWCNT evolve differently: in contrast with 9,0 SWCNT,
which exhibits favorable conditions for photoexcitation, the 13,0 and 5,0 SWCNTs do not show a lowering
of the lowest excited states. This behavior is discussed by taking into account electron—electron interactions
as considered in the framework of the CIS procedure. Furthermore, the 13,0 and 5,0 SWCNTs present
forbidden transitions for the lowest excitations and its first dipole-allowed transitions are at 0.9–1.0 and
1.4–1.6 eV, respectively. In contrast, 9,0 SWCNT allows excitations by photon at less than 0.4 eV as the
length of the nanotube tends to infinite. Excitons appear more bounded, energetically and spatially, in the
13,0 than in the 9,0 and 5,0 SWCNTs. |